1. Field of the Invention
The present invention relates generally to fiber optic cables. More particularly, the present invention relates to extended length side-emitting fiber optic cables.
2. Background of the Invention
Side-emitting optical cables are available which can accept light from a point source and distribute it along a linear path for area illumination. One variation of side-emitting optical cables makes use of a series of notches in the optical fiber, each notch having a surface angled to the axis of the fiber. The angled surface reflects a portion of the light out of the fiber, making it available for use, either to be coupled into several output fibers, or more generally, into free space. Alternatively, an optical fiber may contain regions having differing indices of refraction so as to refract light out of a side of the optical fiber at various locations along the fiber""s length. Another method makes use of reflective or refractive particles distributed throughout an optical fiber, causing scattering of light. The scattered light escapes through the side of the fiber.
Yet another method of providing side-emitting fibers is to make use of all-plastic fibers which have an outer jacket which allows transmission of light radiated continuously along the length of the fiber. This type of fiber is provided for markets such as landscaping, architecture and pool lighting, for example.
Since plastic fibers are flexible in comparison to silica fibers, they may be manufactured in larger diameters without increasing risk of damage from bending. Further, plastic fibers generally have a large numerical aperture (NA) compared to silica fibers. This combination makes plastic fibers well suited to use with large or diffuse light sources which might be difficult to couple into small, low NA silica fibers. Thus, plastic fibers are a good choice when working in area lighting applications which do not require the precision available with silica fibers.
Plastic fibers have two important limitations, however, when used in outdoor applications. The first is that they tend to have relatively high attenuation in the visible light spectrum, on the order of hundreds or thousands of dB/km, compared to available silica core fibers having attenuations below one dB/km. The second limitation results from a spectrum dependent transmittance. When white light is injected into a conventional plastic fiber, red light is absorbed at a greater rate than shorter wavelength blue light. Thus, as the light travels along the fiber the light becomes more and more blue, resulting in a blue tint to light emitted from the side of the fiber at points furthest from the light source. This has the obvious disadvantage that over long distances, side-emitting plastic fibers are not well suited to situations requiring a true color light source. These two limitations result in an effective length for an all-plastic illumination fiber of less than about 100 feet.
While silica core fibers are well adapted to providing long transmission distances, having low attenuation and good spectral performance, they are not well suited to use as side-emitting fibers and are thus, not presently a good choice for area lighting applications.
The present invention addresses the needs identified above by providing a hybrid optical fiber cable, which includes a first side-emitting illumination optical fiber and a second side-emitting illumination optical fiber. A first end of the second side-emitting illumination optical fiber is displaced longitudinally from a first end of the first side-emitting illumination optical fiber. A transmitting optical fiber is optically coupled to the first end of the second side-emitting illumination optical fiber to produce good spectral performance and long transmission distances in a fiber cable which provides side-emitted light.
In another embodiment, a strength member is disposed around the optical fibers and allows light emitted by the side-emitting illuminating optical fibers to pass therethrough.
In yet another embodiment, an outer jacket is disposed around the optical fibers and the strength member. As with the strength member, the outer jacket allows light emitted by the side-emitting illuminating optical fibers to pass therethrough.
In another embodiment, the first end of the second side-emitting optical fiber is displaced longitudinally from the first end of the first side-emitting optical fiber by a distance greater than about 25 feet.
In another embodiment of the present invention the first end of the second side-emitting optical fiber is displaced longitudinally from the first end of the first side-emitting optical fiber by a distance such that the optical fiber cable retains sufficient chromaticity to obtain a predetermined lighting effect.
In an alternate embodiment of the present invention, a light source is optically coupled into the first end of the first side-emitting illumination optical fiber and into the first low attenuation optical fiber and optically coupled via the first low attenuation optical fiber to the first end of the second side-emitting illumination optical fiber.
Another embodiment includes a third side-emitting illumination optical fiber. A first end of the third side-emitting fiber is displaced longitudinally from the first end of the first side-emitting illumination optical fiber and the first end of the second side-emitting illumination optical fiber. A second low attenuation optical fiber is optically coupled to the first end of the third side-emitting illumination optical fiber. This embodiment may further include a light source optically coupled into the first end of the first side-emitting illumination optical fiber and into the first and second low attenuation optical fibers and optically coupled via the first low attenuation optical fiber to the first end of the second side-emitting illumination optical fiber and via the second low attenuation optical fiber to the first end of the third side-emitting illumination optical fiber.
Additionally, the present invention may be embodied in an illumination device, including a first side-emitting illumination optical fiber having a first end optically coupled to a light source, a second side-emitting illumination optical fiber, a first end thereof displaced longitudinally from the first end of the first side-emitting illumination optical fiber, and a low attenuation optical fiber, a first end thereof optically coupled to the light source, a second end thereof optically coupled to the first end of the second side-emitting illumination optical fiber.